Question

START TRANSACTION;
SELECT @A:=SUM(salary) FROM table1 WHERE type=1 FOR UPDATE;
UPDATE table2 SET summary=@A WHERE type=1;
COMMIT;

I have taken this query directly from the MySQL official documentation and I am wondering certain aspects (I added myself the FOR UPDATE). Supposing that there is another user that already has a lock on the record. In that case the update should fail. If that were to happen I am wanting for sure a way to display to my user a message box that will alert the user that there is a lock on the record. How might I get that information back into PHP? Also wouldn’t this query roll back by default or do I specifically need to add the ROLLBACK?

Thanks in advance!

Answer 1

You have to reset the value of autocommit variable to OFF / 0 / FALSE.
Try this:

SET SESSION autocommit = 0;
START TRANSACTION;
SELECT @A:=SUM(salary) FROM table1 WHERE type=1 FOR UPDATE;
UPDATE table2 SET summary=@A WHERE type=1;
COMMIT;